Purification of gases in synthesis gas production process

ABSTRACT

A modified purifier process, includes supplying a first stream of a feed gas containing hydrogen and nitrogen in a mol ratio of about 2:1, and also containing methane and argon, then cryogenically separating the feed gas into the following:
         f) a second stream of synthesis gas containing hydrogen and nitrogen in a mol ratio of about 3:1,   g) waste gas containing principally nitrogen, and also containing some hydrogen and all of the methane supplied in the first stream,
 
and splitting the waste gas into:
   h) a third stream of hydrogen/nitrogen gas   i) a fourth stream of high concentrated nitrogen   j) a fifth stream of methane rich gas, to be used as fuel.
 
The combined second and third streams typically are passed to an ammonia synthesis process.

This application is a continuation-in-part of pending U.S. application Ser. No. 12/586,350, filed Sep. 22, 2009, which is a regular application converted from Provisional application Ser. No. 61/192,556, filed Sep. 22, 2008.

BACKGROUND OF THE INVENTION

The invention relates generally to purification of feed gas used for the manufacture of ammonia, and more particularly to improvements in processing of feed gas from which hydrogen rich ammonia synthesis gas and waste gas are derived. The invention specifically concerns treatment of the waste gas to derive useful gas streams, one of which is hydrogen/nitrogen rich, another is nitrogen rich, and another is methane rich. In the prior purifier process, synthesis gas is separated from the waste gas, which contains excess nitrogen from the feed gas, a small amount of hydrogen, all of the incoming methane and about 600 of the incoming argon. Such waste gas is typically utilized as fuel in a primary reformer.

Improvements in treatment of the waste gas are needed for enhanced overall process efficiency.

SUMMARY OF THE INVENTION

It is a major object of the invention to provide improvements in treatment of such waste gas, as will be seen. Basically, the improved process of the invention derives three product streams from the waste gas, one of which is hydrogen/nitrogen rich, another is basically nitrogen rich, and another which is methane rich, with a higher heating value than in processes employed so far, more suitable for use as a fuel, with less nitrogen going up the stack and eventually full recovery of hydrogen. The overall process includes the steps:

-   -   1) supplying a first stream of a feed gas containing hydrogen         and nitrogen in a MOL ratio of about 2/1, and also containing         methane and argon,     -   2) cryogenically separating the feed into the following:         -   a) a second stream of a synthesis gas containing hydrogen             and nitrogen in a MOL ratio of about 3/1,         -   b) waste gas containing principally nitrogen, and also             containing substantially all of the methane supplied in the             first stream,     -   3) and splitting the waste gas into:         -   c) a third stream of hydrogen/nitrogen gas,         -   d) a fourth stream of highly concentrated nitrogen,         -   e) a fifth stream of methane rich gas, useful as fuel.

In that overall process, the second, third, fourth and fifth streams are typically delivered as product streams; and the second plus third product streams of synthesis gas may be advantageously delivered to an ammonia synthesis process.

Another object is to provide the split into a third, fourth and fifth streams, through cryogenic separation in such manner that

-   -   a) the amount of hydrogen of the third stream equals the         hydrogen content of the waste gas     -   b) the amount of methane of the fifth stream equals the methane         of the waste gas.

Accordingly, the prior “Purifier” process is modified and improved through these measures, in that

-   -   a) all incoming hydrogen is completely recovered towards         synthesis gas     -   b) the heating value of the methane rich gas is increased,         typically from 165 BTU/SCF (LHV) to about 700 BTV/SCF (LHV).         The methane rich gas is used as fuel and increased heating value         improves the combustion.

These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which:

DRAWING DESCRIPTION

FIG. 1 is a diagram showing the separation of a feed gas into synthesis gas and waste gas as in the Purifier process.

FIG. 2 is a diagram showing the additional split of the waste gas into hydrogen/nitrogen gas, nitrogen rich gas and methane rich gas,

FIGS. 3 and 4 show detailed processes.

DETAILED DESCRIPTION

In FIG. 1, feed gas, such as hydrogen, nitrogen, argon and methane is fed at 10 to a purification or separation process 11. The feed gas typically has an H/N ratio of about 2. Separated hydrogen is fed at 12 (in a stream with a H/N ratio of about 3) from the process 11, and delivered for example as synthesis gas to a conversion process producing ammonia. Separated “waste” gas is fed at 13 from the process 11, and contains nitrogen, methane, and about 60% if the incoming argon at 10, usable as a low grade fuel for combustion and heating, for example to the primary reformer or to a boiler. The typical heating value of the waste gas 13 is approximately 160 BTU/SCF (LHV). See in this regard U.S. Pat. No. 3,442,613 to Grotz.

In a preferred and improved prior Purifier process as represented in FIG. 2 and in more detail in FIG. 3 feed gas is delivered at 110 to a cryogenic separation process indicated generally at 111. Synthesis gas is withdrawn from the process at 112. Nitrogen rich gas and methane rich gas are separated in the process and delivered as streams 113 and 114 respectively. The methane rich gas 114 is typically used as a (high grade) fuel for instance in the primary reformer upstream of 111.

Referring in detail to process 111 in FIG. 3 coldbox 115 a and columns 130 and 140 are additions to an existing coldbox 115 with an existing column 116. The streams 110, 112 c and 131 flow through the existing coldbox or refrigerated heat exchanger 115 for heat exchange as shown via coils 110 a, 110 b, 112 a and 126 a. As in the purifier process expander C4 provides refrigeration between coils 110 a and 110 b. An existing separation column 116 receives the refrigerated feed via line 117 and synthesis gas is taken from the top of this column and passed through the existing top mounted refluxed condenser 119. Synthesis gas is taken overhead via line 121 and passed to coil 112 a in the existing box 115 for delivery at line 112 c.

Waste gas is taken from the bottom of the existing column 116 and is passed via line 122 to the existing Joule Thompson valve 123. A typical pressure drop through the JT valve is 300 to 350 psi.

Cooled waste gas then passes via line 125 to provide refrigeration for the existing condenser 119. It passes through line 126 and coil 126 a in the existing coldbox 115 for delivery via line 131 to coil 114 a in an additional coldbox 115 a and exits via line 131 b as feed to an additional second column 130. Column 130 is provided with a top mounted refluxed condenser 135.

Methane rich gas leaves the bottom of column 130 via line 133 to flow to coil 145 a in the additional coldbox 115 a to deliver at line 134. If needed, the pressure of the methane rich gas is boosted in a single stage blower C1 and methane rich gas is delivered at 114.

Overhead gas is taken via line 132 to a third additional column 140. The separation in column 130 is such that all of the incoming hydrogen via line 131 b but none of the incoming methane via line 131 b goes overhead via line 132.

The additional third column 140 is provided with a top mounted refluxed condenser 145. Nitrogen rich gas leaves the bottom of column 140 via line 143 to flow to coil 113 a in the additional coldbox 115 a, and to deliver at line 113. Nitrogen rich gas (typically 97⁺% nitrogen, with the remainder being Argon) may be rejected to the atmosphere.

Overhead gas from the additional column 140 is taken via line 142 to coil 140 a in the additional coldbox 115 a to deliver at line 146. The separation in column 140 is such that all of the incoming hydrogen via line 132 goes overhead at column 140. Hydrogen/nitrogen delivered at line 146 is recompressed in compressor C2 and combined with the synthesis gas at line 112 c, and is delivered at line 112.

Refrigeration for the refluxed condensers 135 and 145 is provided by a refrigeration compressor C3. The discharge of compressor C3 delivers via line 151 to coil 150 a in the additional cold box 115 a. The cold refrigerant leaves via line 152 and is expanded via valve 153 to line 154. Refrigerant to refluxed condenser 135 is delivered via line 155; refrigerant to refluxed condenser 145 is delivered via line 156. Refrigerant returns from the refluxed condenser 135 via line 157 and from refluxed condenser 145 via line 158. The combined refrigerant returns via line 159 into coil 150 b in the additional coldbox 115 a, and leaves via line 160 to the suction of the refrigerant compressor C3.

Following data are representative for FIG. 3

Feedgas Synthesis gas waste gas NZ rich gas CH9 rich gas Stream # 110 112c 146 112 131b 113 134 Temp ° F. 40 35 35 34 30 35 35 pressure psia 399 348 40 348 40 15 25 Comp. MOL % H2 66.1 76.2 20.5 73.8   5.5 — — Comp. MOL % N2 31.0 23.6 79.5 26.0   75.5 96.8 28.9 Comp. MOL % Ar 0.5 0.2 — 0.2   2.1 3.2 2.5 Comp. MOL % CH₄ 2.4 — — —   16.9 — 68.6 100 100 100 100 100  100 100 LHV BTU/SCF — — — —  17- — 625 Temperatures T1 = −286° F. T2 = −295 T5 = −284 T6 = −304 T8 = −325 T10 = −321 T13 = 308 Pressures P1 = 385 psia P2 = 22 P3 = 20 P4 = 15 P5 = 172

For a completely new (grass roots) design the coldboxes 115 and 115 a of FIG. 3 can advantageously be combined into one coldbox 180, and the expander C4 can be eliminated, as shown in FIG. 4.

Referring in detail to process 211 in FIG. 4 the streams 110, 112 c, 113 and 134 flows through a coldbox or refrigerated heat exchanger 116 for heat exchange as shown via coils 110 a, 112 a, 113 a and 114 a. A first separation column 116 receives the refrigerated feed via line 117 and synthesis gas taken from the top of this column and passed through a top mounted refluxed condenser 119. Synthesis gas is taken overhead via line 121 and passes to coil 112 a in box 180 for delivery at line 112 c.

Waste gas is taken from the bottom of the column 116 and is passed via line 122 to the Joule Thompson valve 123. A typical pressure drop through the JT valve is 300 to 350 psi.

Cooled waste gas then passes via line 125 to provide refrigeration for the condenser 119. It passes through line 126 and coil 126 a in the coldbox 180 for delivery via line 131 as feed to a second column 130. Column 130 is provided with a top mounted refluxed condenser 135.

Methane rich gas leaves the bottom of column 130 via line 133 to flow to coil 114 a in coldbox 180 to deliver at line 134. If needed, the pressure of the methane rich gas is boosted in a single stage blower C1 and methane rich gas is delivered at 114.

Overhead gas is taken via line 132 to a third column 140. The separation in column 130 is such that all of the incoming hydrogen via line 131 but none of the incoming methane via line 131 goes overhead via line 132.

Third column 140 is provided with a top mounted refluxed condenser 145. Nitrogen rich gas leaves the bottom of column 140 via line 143 to flow to coil 113 a in coldbox 180, and to delivery at line 113. Nitrogen rich gas (typically 97% nitrogen, with the remainder being Argon) may be rejected to the atmosphere.

Overhead gas from column 140 is taken via line 142 to coil 140 a in coldbox 115 to deliver at line 146. The separation in column 1450 is such that all of the incoming hydrogen via line 132 goes overhead at column 140. Hydrogen/nitrogen delivered at line 146 is recompressed in compressor C2 and combined with the synthesis gas at line 112 c, and is delivered at line 112.

Refrigeration for the refluxed condensers 135 and 145 is provided by a refrigeration compressor C3. The discharge of compressor C3 delivers via line 151 to coil 150 a in coldbox 180. The cold refrigerant leaves via line 152 and is expanded via valve 153 to line 154. Refrigerant to refluxed condenser 135 is delivered via line 155; refrigerant to refluxed condenser 145 is delivered via line 156. Refrigerant returns from the refluxed condenser 135 via line 157 and from refluxed condenser 145 via line 158. The combined refrigerant returns via line 159 into coil 150 b in coldbox 180, and leaves via line 160 to the suction of the refrigerant compressor C3.

The following data are representative for FIG. 4.

Feed N2 Rich CH4 Rich gas Synthesis gas gas gas stream # 110 112c 146 112 113 134 Temp. ° F 38 39 32 38 40 30 psia 391 379 18 379 17 17 comp. mol % H2 67.3 75.7 31.3 74.7 — — comp. mol % N2 29.7 24.0 68.7 25.0 97.9 22.0 comp. mol % Ar 0.6 0.3 — 0.3 2.1 5.3 comp. mol % CH4 2.4 — — — — 72.7 100 100 100 100 100 100 LHV BTU/SCF — — — — — 660

The presentation of the coldboxes 115, 115 a and 180 in FIG. 3 and FIG. 4 is schematic and each coldbox is characterized by the following:

-   -   1) Heat is exchanged between the flowing process streams, and         the temperatures change accordingly as indicated. The heat         exchange between the warm and the cold streams is in balance.     -   2) The heat exchangers and columns are embedded in one common         box, providing cold insulation to prevent ingression of heat to         the equipment. The insulation side of the cold box interior has         one common identical stagnant temperature, for the whole box         interior.     -   3) The presentation in FIG. 3 and FIG. 4 indicates that heat         exchange occurs directly between the warm and cold streams,         inside the heat exchange device.     -   4) Accordingly, the cold box interior maintains, throughout the         entirety of the gas purification process, the same temperature         at which the indicated streams are passed through the cold box         interior, after the cryogenic separation.

The parameters, upstream of the coldbox as presented, are to be adjusted as to maintain the feed gas to the coldbox per FIG. 3 and FIG. 4 line 110. 

1. A gas purification process, comprising the steps, 1) supplying a first stream of feed gas containing hydrogen and nitrogen in a MOL ratio of about 2/1, and also containing methane and argon, 2) cryogenically separating the feed gas into the following a) a second stream of synthesis gas containing hydrogen and nitrogen in a MOL ratio of about 3/1, b) a waste gas stream containing principally nitrogen, and also containing substantially all of the methane supplied to the first stream, 3) and splitting the waste gas stream into: c) a third stream of hydrogen/nitrogen gas d) a fourth stream of nitrogen (97 MOL %⁺) with the remainder being argon e) a fifth stream of methane rich gas useful as fuel, 4) there being coldbox means having a common cold interior and wherein said first, second, third, fourth and fifth streams, and said waste gas stream after cooling thereof are passed through said common cold interior or interiors of said coldbox means, said interior or interiors effectively maintaining throughout the entirety of the gas purification process the same temperature at which said first, second, third, fourth and fifth streams being passed through the coldbox means interior or interiors after said cryogenically separating, 5) then delivering said second stream of synthesis gas to an ammonia synthesis process, 6) and delivering said third stream of hydrogen/nitrogen gas to an ammonia synthesis process.
 2. The process of claim 1 including delivering said second, third, fourth and fifth streams as product streams.
 3. The process of claim 2 wherein said second, third, fourth and fifth streams are passed in generally parallel relation through the coldbox means.
 4. The process of claim 1 including: i) providing a Joule Thompson valve through which the waste gas stream is passed, to drop the gas pressure and produce refrigeration, and ii) then passing the waste gas to a heat exchanger for cooling of said second stream, iii) and passing the waste gas to said coldbox means.
 5. The process of claim 1 that provides: i) 100% hydrogen recovery of the incoming feed gas towards the synthesis gas, ii) enhanced heating value of the methane rich gas, to be used as fuel.
 6. The process of claim 1 wherein the ammonia production is increased by 2 to 3% for the same natural gas for feed plus fuel at the plant battery limits.
 7. The process of claim 1 wherein said second and third streams are combined and delivered to the same ammonia synthesis process.
 8. The process of claim 1 including a first separator column receiving feed gas and operating to separate synthesis gas passed through a first top mounted reflexed condenser, the separated synthesis gas then flows to and through the coldbox means for delivery as product.
 9. The process of claim 8 including a waste gas delivery from the bottom of the first separator column, for delivery and flow to a second separator column via i) cooling stage whereby the delivery from the cooling stage is passed to said first condenser acting as refrigerant therefor, ii) and via the coldbox means.
 10. The process of claim 9 wherein the second separator operated to separate methane rich gas leaving the column bottom to flow through the coldbox means for delivery as product methane rich gas.
 11. The process of claim 10 wherein the second separator column is operated to separate all incoming hydrogen delivered free of methane via a second condenser stage at the top of the second column for delivery to a third separator column.
 12. The process of claim 11 whereby the third column operates to deliver synthesis gas via a third condenser stage at the column top, the delivered synthesis gas then flowing via the coldbox means as product synthesis gas, and nitrogen rich product gas delivered from the third column bottom, and via flow through the coldbox means, as product.
 13. The process of claim 12 wherein coolant is supplied to flow through said coldbox means, and then through said second and said third condenser stages, in sequence.
 14. The process of claim 13 including a coolant supply compressor from which coolant flows through the coldbox means, and then to the two condenser stages, and to which coolant from said condenser stages is returned via the coldbox means, to the compressor.
 15. The process of claim 1 wherein said coldbox means comprises an existing coldbox and an added coldbox separate from the existing coldbox.
 16. A purifier process includes supplying a first stream of a feed gas containing hydrogen and nitrogen in a mol ratio of about 2:1, and also containing methane and argon, then cryogenically separating the feed gas into the following: a) a second stream of synthesis gas containing hydrogen and nitrogen in a mol ratio of about 3:1, b) waste gas containing principally nitrogen, and also containing some hydrogen and all of the methane supplied in the first stream, and splitting the waste gas into: c) a third stream of hydrogen/nitrogen gas d) a fourth stream of high concentrated nitrogen e) a fifth stream of methane rich gas, to be used as fuel, The combined second and third streams typically are passed to an ammonia synthesis process. 